1. Field of the Invention
The present invention relates to a honeycomb structure for use in a filter or the like to collect particulate matters included in an exhaust gas of a diesel engine or a gasoline engine.
2. Description of Related Art
In an exhaust gas of a diesel engine or a gasoline engine such as a gasoline direct injection (GDI) engine, a particulate matter (PM) is included. This PM is mainly composed of carbon fine particles such as soot, and it is recognized to be a carcinogen, so that it is necessary to prevent the PM from being discharged to the atmosphere, and hence strict emission regulations have been imposed thereon.
To cope with such strict emission regulations, a large number of investigations have been performed to decrease an amount of the PM emission, but there is a limit to the decrease of the amount of the PM emission by improvement of a burning technology, and at present, the disposing of a filter in an exhaust system is only one effective means for decreasing the amount of the PM emission.
As the filter for collecting the PM, a wall flow type filter using a honeycomb structure has broadly been used because a high PM collecting efficiency can be obtained while suppressing a pressure loss to an allowable range. The honeycomb structure for use in the wall flow type filter has porous partition walls to define and form a plurality of cells extending from an inlet end surface on an inlet side of the exhaust gas to an outlet end surface on an outlet side of the exhaust gas. In this honeycomb structure, plugging portions are disposed to plug open end portions on an outlet end surface side of predetermined cells and open end portions on an inlet end surface side of the remaining cells, whereby the filter having a high PM collecting efficiency can be obtained.
That is, in the honeycomb structure in which the plugging portions are disposed in this way, the exhaust gas which has flowed into the cells from the inlet end surface passes the partition walls, and then flows out of the cells from the outlet end surface. When the exhaust gas passes the partition walls, the partition walls function as filter layers, so that the PM included in the exhaust gas is collected.
Additionally, to continuously use such a filter for a long period of time, the filter needs to be periodically subjected to a regeneration treatment. That is, for the purpose of decreasing the pressure loss increased due to the PM deposited in the filter with an elapse of time to return a filter performance to an initial state, the PM deposited in the filter needs to be burned by a high-temperature gas and removed. Furthermore, during this regeneration, high heat stress is generated in the filter due to the burning heat of the PM, and hence the filter is damaged sometimes.
Heretofore, as a measure for preventing such damage on the filter, it has been suggested that the whole filter is not manufactured as one honeycomb structure, but a plurality of segments having a honeycomb form (honeycomb segments) are bonded to obtain the honeycomb structure for the filter. Specifically, the plurality of honeycomb segments are integrally bonded to one another by a bonding material which has a low elastic modulus and is easily deformed to obtain a segment structure, whereby the heat stress applied to the honeycomb structure during the regeneration is scattered and alleviated to intend enhancement of heat shock resisting properties.
When the honeycomb structure of such a segment structure is used, the filter having high thermal shock resistance properties can be obtained, but on the other hand, a new problem inherent in the honeycomb structure of the segment structure occurs. That is, in the filter in which the honeycomb structure of the segment structure is used, there is the problem that movement (shift) easily occurs in the honeycomb segments constituting the honeycomb structure due to vibration or a pressure of the exhaust gas (hereinafter referred to as an exhaust pressure) during the use of the filter.
As one of means for solving this problem, in Patent Document 1, it has been suggested that a flatness of an outer wall as a bonding surface of honeycomb segments is set to a specific value, thereby enhancing a bonding strength between the honeycomb segments. Furthermore, in Patent Document 2, it has been suggested that a honeycomb segment in which an area of an inlet side end surface is smaller than an area of an outlet side end surface is used, and a bonding width of a bonding material of the inlet side end surface is larger than a bonding width of the bonding material of the outlet side end surface, thereby heightening a structural strength.    [Patent Document 1] JP-A-2001-138416    [Patent Document 2] WO 2008/096502